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Viscosity and Voluminosity of Bovine Milk Casein Micelles

Department of Biochemistry
Department of Food Science and Industries, University of Minnesota, St. Paul 55101

ABSTRACT

We have surveyed methods for determining the voluminosity and hydration of casein micelles from viscosity measurements. Results by previous workers who applied the Einstein viscosity equation to concentrated casein solutions or used the empirical Eilers equation for concentrated solutions are reviewed. A more rigorous and reliable procedure is to determine intrinsic viscosities by extrapolating relative viscosities to infinite dilution. Several experimental procedures for performing such dilutions, so as to make the intrinsic viscosity reflect the properties only of the casein micelles and not of soluble proteins, are discussed and applied to several pooled milk samples. The voluminosity of casein micelles determined by intrinsic viscosity is about 4.4 ± .3 ml/g at 25 C; the Einstein and Eilers equations overestimate and underestimate this, respectively. The hydration determined from voluminosity measurements is 3.7g water per gram protein. This is about twice the hydration measured from the water content of casein micelles pellets. The discrepancy may arise from squeezing out of the water from the pellets by the high hydrostatic pressures in the ultracentrifuge. Addition of 10% sucrose to skimmilk reduces the voluminosity by about 20% while further sucrose addition has little effect.